Water is the source of all life on earth.
The distribution of water, however, is quite varied; many locations
have plenty of it while others have very little.
Water exists on earth as a solid (ice), liquid or gas (water vapor).
Oceans, rivers,
clouds, and rain, all of which contain water, are in a
frequent state of change (surface
water evaporates,
cloud water precipitates, rainfall infiltrates
the ground, etc.). However,
the total amount of the earth's water
does not change.
The circulation and conservation
of earth's water is called the "hydrologic cycle".
The Hydrologic Cycle module has been organized
into the following sections: